The In-Plane Switching (IPS) technology is a wide-viewing angle technology applied to liquid crystal display (LCD), which is widely used in manufacturing LCD television to improve the viewing angle effectively. As shown in FIG. 1, an electrode structure of such LCD television includes first electrodes 101 and second electrodes 102, and the liquid crystal molecules in the IPS-mode display are arranged in parallel with each other when no voltage is applied to the electrode structure.
The Advanced Super Dimension Switch (ADS) technology is a core technology of planar-electric-field wide-viewing angle, in which a multidimensional electric field is formed by an electric field generated from the edges of the slit electrodes located in a same plane and an electric field generated between the slit electrode layer and the plate electrode layer, so that all the liquid crystal molecules located among the slit electrodes and located right over the slit electrodes in the liquid crystal cell can rotate. Thus, both the operating efficiency and the transmission efficiency of the liquid crystal material are improved. With the ADS technology, the TFT-LCD product has the advantages of improved picture quality, high resolution, high transmittance, low power consumption, wide viewing angle, high aperture ratio, low chromatic aberration, no squeezing water ripples, etc. For different applications, the improved ADS technologies include high-transmittance I-ADS technology, high-aperture ratio H-ADS technology and high-resolution S-ADS technology, and so on. As shown in FIG. 2, the electrode structure of ADS-mode LCD screen comprises a color filter (CF) substrate 201, array substrate including a plural of thin film transistors (TFTs) (hereinafter referred as thin film transistor substrate or TFT substrate) 205 disposed opposite to the CF substrate 201, and a first electrode 204, an insulating layer 203 and second electrodes 202, which are disposed between the CF substrate 201 and the TFT substrate 205 in this order, from bottom to top. Similar to that in the IPS-mode LCD screen, the liquid crystal molecules in the ADS-mode LCD screen are also arranged in parallel with each other when no voltage is applied to the electrode structure.
In order to enable the LCD screen to forming images, it is necessary to provide a polarizing layer to the upper side and the lower side of the liquid crystal cell (including the TFT substrate and the CF substrate opposite to each other, and the liquid crystal layer between the TFT substrate and the CF substrate), respectively, that is, a first polarizing layer located at the TFT substrate side and a second polarizing layer located at the CF substrate side. In both the IPS-mode and the ADS-mode LCD screens, the transmission axis of the first polarizing layer is perpendicular to the transmission axis of the second polarizing layer, and parallel to the absorption axis of the second polarizing layer. When not being applied with power, the polarized light transmitting through the first polarizing layer is absorbed by the second polarizing layer completely. However, light leakage may occur in both the IPS-mode and the ADS-mode LCD screens. For example, in the case that the azimuth angle of the incident light with respect to the polarizing layer is 45°, and the polar angle of the incident light with respect to the polarizing layer is 60°, the light leakage is most severe. The principle of the light leakage will be described with reference to FIG. 3. The light leakage results from that, when the light transmits along above directions, the transmission axes of the polarizing layers at the upper and the lower sides of the liquid crystal cell are not orthogonal to each other, and the direction of the light axis of the liquid crystal molecules is changed to some extent.
In the following description, such change will be described by using the Poincare Sphere. The Poincare Sphere is a mathematic model invented by the Jules Henri Poincaré in 1892. In the stokes space, it is introduced the normalized radius S0=1; S02=S12+S22+S32, wherein S1, S2 and S3 correspond to the coordinates x, y and z of the solid sphere respectively, and are used to represent the polarization state of the light. Any one point on the sphere corresponds to a certain polarization state of the completely polarized light with unit intensity. The points at the equator of the Poincare Sphere represent linearly polarized light, the upper and lower poles correspond to right-circularly polarized light and left-circularly polarized light respectively, and other points correspond to elliptically polarized light. The polarized lights corresponding to the points at the same longitude on the Poincare Sphere have identical polarized directions.
As shown in FIG. 3, when the incident light is incident into the first polarizing layer at a polar angle of 60 degree and an azimuth angle of 45 degree, the transmission axis of the first polarizing layer on the TFT substrate side will deflect to the point T in the figure, while the absorption axis of the second polarizing layer on the CF substrate side will deflect to the point A in the figure. Since the transmission axis of the first polarizing layer and the absorption axis of the second polarizing layer do not consist with each other any more, and the light axis of the liquid crystal molecules also deflect by a certain angle at the same time, the polarized light transmitting through the first polarizing layer cannot be absorbed by the second polarizing layer completely, and thus the light leakage occurs.
Nowadays, the wide-viewing angle optical compensation for the ADS-mode and IPS-mode LCD screens has been widely applicable. However, the existing optical compensation methods each only works for a certain display mode of the LCD screen. The display modes of the ADS-mode and IPS-mode LCD screens may be classified into an E mode, an O mode and a multi domain mode. In an E-mode LCD screen, the direction of the slow axis of the liquid crystal molecules consists with the direction of the transmission axis of the first polarizing layer on the TFT substrate side. In an O-mode LCD screen, the direction of the slow axis of the liquid crystal molecules consists with the direction of the transmission axis of the second polarizing layer on the CF substrate side. The multi domain mode is a mixed mode formed by the E mode and the O mode. For an E-mode LCD screen, the compensation film should be disposed between the TFT substrate and the first polarizing layer on the TFT substrate side. For an O-mode LCD screen, the compensation film should be disposed between the CF substrate and the second polarizing layer on the CF substrate side. For the multi domain mode, there is still no any proper optical compensation method nowadays. The reasons are presented as follows.
In the E-mode liquid crystal display screen, the transmission axis of the first polarizing layer and the slow axis of the liquid crystal molecule will deflect to the point A shown in FIG. 3 since the slow axis of the liquid crystal molecule consists with the transmission axis of the first polarizing layer in direction. In this case, the compensation may be achieved only by performing compensation such that the polarization direction of the polarized light incident from the TFT substrate side is changed from the point T to the point A before the polarized light incident from the TFT substrate side transmits through the liquid crystal material. The liquid crystal material without being applied with voltage will not change the polarization direction of the polarized light, which enables the polarized light having transmitted through the liquid crystal material to be directly absorbed by the second polarizing layer on the CF substrate side. The optical-compensated structure for the E-mode liquid crystal display screen is shown in FIG. 4.
In the E-mode liquid crystal display screen, the optical axis of the liquid crystal molecule will deflect to the point T shown in FIG. 3. The polarization direction of the polarized light incident from the TFT substrate side still directs to the point T after the polarized light incident from the TFT substrate side transmits through the liquid crystal material. The liquid crystal material without being applied with voltage will not change the polarization direction of the polarized light. Accordingly, if the polarization direction of the polarized light is compensated to change from the point T to the point A after the polarized light transmits through the liquid crystal material, it can ensure that the light incident to the second polarizing layer may be completely absorbed. The optical-compensated structure for the O-mode liquid crystal display screen is shown in FIG. 5.
The multi domain-mode liquid crystals of IPS mode and ADS mode liquid crystal display screens may be classified into two types. In one type of the liquid crystal, the liquid crystal molecules have two initial orientations perpendicular to each other, and may be obtained by way of the optical orientation. In the other type of the liquid crystal, the liquid crystal molecules have a unique initial orientation and may be obtained by ways of the ordinary film orientation and the optical orientation, in which there is a certain angle between the electrodes. Despite of the type of the multi domain-mode liquid crystal, a method for optically compensating the light leakage has not been proposed for the LCD with multi domain-mode liquid crystal since it is a mixture mode of E- and O-modes.
In general, the methods for optically compensating the light leakage are different between the LCD of E-mode and that of O-mode at present. Furthermore, it has not been proposed an optical compensation method for the LCD with multi domain-mode liquid crystal formed by E-mode and O-mode liquid crystal.